NPSH Available & Keep the Pump Clear of Cavitation
Protects surface pumps
If the head at the suction falls below what the pump needs it cavitates and self-destructs — enter the atmospheric head, suction lift, friction loss and vapor-pressure head to get the NPSH available in metres.
Enter your suction side
Next: compare this against the pump's NPSH required on its curve — you need NPSHa to exceed NPSHr by at least 0.5–1 m of margin, or the pump will cavitate.
NPSHa = atmospheric head − suction lift − friction loss − vapor pressure head.
NPSH available — key facts
- Formula
- NPSHa = atm − lift − friction − vapor
- Atmospheric head
- ≈ 10.3 m at sea level
- Altitude effect
- −1 m per ~900 m of elevation
- Must exceed
- the pump's NPSH required
- Safe margin
- ≥ 0.5–1 m above NPSHr
- Vapor head 10 °C
- ≈ 0.12 m
- Below NPSHr →
- cavitation, pitting, lost flow
- Privacy
- Runs in your browser; nothing uploaded
Cavitation eats a pump from the inside out
When the pressure at a pump's suction drops below the water's vapour pressure, the water boils into bubbles that collapse the instant they hit higher pressure in the impeller. Each tiny implosion hammers the metal, so a cavitating pump loses flow, rattles like it is pumping gravel, and pits its impeller until it fails. The defence is simple in principle: keep the head available at the suction — NPSH available — comfortably above what the pump requires.
This tool returns the NPSH available in metres from the atmospheric head, suction lift, friction loss and vapor-pressure head, so you can check a surface pump on a deep or falling source before it ever runs. Pair it with the Total Dynamic Head, Irrigation Pump Power and Pipe Size tools for a complete pump design.
Stop cavitation early
Check NPSHa before the pump pits its impeller.
Site the pump right
See how lift and altitude squeeze your margin.
Match the pump curve
Compare available against the required NPSH.
Diagnose a noisy pump
Test whether suction conditions explain the rattle.
Frequently Asked Questions
How is NPSH available calculated?+
The tool uses NPSHa = atmospheric head − suction lift − suction friction loss − vapor-pressure head, all in metres of water. Atmospheric head is the pressure pushing water up the suction line, the suction lift is how far the pump sits above the water surface, friction loss is what the suction pipe and fittings eat, and the vapor-pressure head is the margin before the water flashes to vapour. Whatever is left is the head available at the pump inlet.
What is NPSH and why does it matter?+
Net Positive Suction Head is the absolute pressure available at the pump suction above the point where water boils. If the available NPSH drops below what the pump requires, the water vaporises into bubbles that collapse violently inside the impeller — cavitation — which causes loss of flow, noise like gravel, vibration and rapid pitting damage. Keeping NPSH available comfortably above NPSH required is how you protect the pump.
How much NPSH available do I need?+
NPSH available must exceed the pump's NPSH required, which comes from the manufacturer's curve at your operating flow, and good practice keeps a margin on top — commonly at least 0.5 to 1 metre, or more for larger pumps. So if the pump needs 3 m at your flow, aim for around 4 m or more available. The tool gives you the available figure; compare it against the required value from the pump curve.
What is the atmospheric head to enter?+
Atmospheric pressure expressed as a height of water is about 10.3 m at sea level, but it falls roughly 1 m for every 900 m of altitude, so a pump at 1,800 m has only about 8 m to work with. Because that head is the ceiling for everything on the suction side, high-altitude installations have much less room before cavitation. Use the value for your elevation, not the sea-level figure, for a realistic answer.
How do I increase NPSH available?+
Lower the suction lift by setting the pump closer to the water surface or below it (a flooded suction is best), cut the friction loss with a shorter, larger, smoother suction pipe and a foot valve with low loss, and reduce the vapor-pressure head by pumping cooler water. You cannot raise the atmospheric head — it is fixed by altitude — so the practical levers are lift and friction. The tool shows the effect of each change at once.
Why does water temperature appear as vapor pressure?+
Warm water boils at a lower pressure, so its vapor-pressure head is higher, which eats directly into the NPSH available. Cold water at 10 °C has a vapor-pressure head of only a few centimetres, but at 60 °C it climbs to nearly 2 m, and near boiling it consumes almost everything. That is why hot-water and high-altitude pumping are the classic cavitation traps, and why the term is subtracted in the formula.
Is suction lift the same as static lift?+
On the suction side, yes — it is the vertical distance the pump has to draw water up, from the water surface to the pump centreline, and it is subtracted because gravity works against the suction. It is not the same as the total static head of the whole system, which also includes the delivery lift on the discharge side. NPSH available concerns only the suction side; the delivery head belongs in the Total Dynamic Head calculation.
Does a submersible or flooded pump avoid this?+
Largely, yes. A submersible pump sits in the water and a flooded-suction surface pump sits below the water level, so the suction lift is zero or negative, which makes NPSH available large and cavitation unlikely. The risk is highest for surface pumps lifting water up from a deep, falling source — a dropping borewell or pond — where suction lift and friction both grow as the water level falls, which is exactly when you should re-check the figure.